As an insert molded article, there is a resin gear with metallic core in which a metallic gear has a part with teeth formed from a synthetic resin to engage with another gear, that is, a metallic sleeve as a metallic core and a synthetic resin tooth part covering the outer periphery of the metallic sleeve to make a power transmission gear lower in weight and noise level.
For further weight reduction, some of the resin gears with metallic core have the tooth part composed of an external member with teeth to engage with another gear (a gear main body in Patent Document 1, a resin part formed from a resin by the second molding in Patent Document 2) and an internal member (an annular member in Patent Document 1, a resin part formed by the first molding in Patent Document 2) positioned between the outer periphery of the sleeve (bush) and the external member (for example, refer to Patent Documents 1 and 2).
These conventional resin gears with metallic core are manufactured by forming the internal member with the metallic sleeve as an insert by injection molding (primary molding), and then forming the external member in another cavity die by injection molding (secondary molding) as illustrated in FIG. 4 of Patent Document 1, for example.
Alternatively, the conventional resin gears with metallic core are manufactured by forming the internal member with the metallic sleeve as an insert by injection molding (primary molding), then removing an integrated object of the metallic sleeve and the resin part from the die, and then loading the object into another die and forming the external member by injection molding (secondary molding) as described in Patent Document 2.